This invention relates to air pollution analyzers and the like which measure constituents in a sample gas stream. More particularly, the invention relates to a cryotrap used for extracting constituents from the sample gas.
In a typical air pollution application, the air sample is passed through a cryotrap which is maintained at the temperature of liquid nitrogen or other liquid cryogens. Constituents in the gas sample are frozen by the low temperature and solidify on the cold surfaces of the trap. After the desired amount of sample gas has passed through the trap, the gas flow path is changed so as to pass a carrier gas through the trap in place of the sample gas. At this time, the trap is heated. The frozen constituents vaporize and are carried away with the carrier gas to an analyzer or other instrument. The now clean trap is ready for another sampling and flushing cycle.
In the cryotraps commonly used, liquid nitrogen is the gas chosen for the freezing step. However, the liquid nitrogen is lost to the atmosphere and a new quantity must be utilized for each operation. For instruments which operate continuously, the cost of liquid nitrogen as the freezing gas is very high.
It is an object of the present invention to provide a new and improved cryotrap which uses a closed cycle cooler for the freezing with no coolant being discharged after each sampling cycle.
A further object of the invention is to provide such a cryotrap with a new and improved construction for obtaining increased efficiency in cooling and heating while enabling a smaller construction and shorter operating cycles.
Earlier mechanical coolers are bulky, heavy, noisy, and have high mechanical vibration. They have relatively short life cycles and do not cool to liquid nitrogen temperature. It is an object of the present invention to use a Stirling closed cycle linear cooler which is relatively lightweight and compact. Features of the Stirling cooler include low conducted and radiated emissions, low mechanical vibration, acoustically quiet operation, and improved lifetime and reliability.
Other objects, advantages, features and results will more fully appear in the course of the following description.